Method of making a hollow mold



Aug. 16, 1960 G. H. BINGHAM, JR

METHOD OF MAKING A HOLLOW MOLD Filed Feb. 20, 1958 1w) Jr MMM a lillll"METHD F G A HOLLOW MOLD George Bingham, Jr., Westminster, Md., assignerto Cambridge Rubber Company, Taneytown, Md., a corporation of MarylandFiled Feb. 20, 1958, Ser. No. 7116,37l

1 Claim. (Cl. 20d-9) rhis invention pertains to a novel method of makingmolds for use in the manufacture of waterproof articles, wherein thewaterproofing material is asynthetic plastic, and relates moreespecially to a method whereby a mold of 'accurate shape and dimensionsmay be produced.

In the manufacture of plastic footwearfor example, but withoutlimitation, boots, it has been proposedY to prepare a hollow mold byiirst building up, upon a conventional boot last of the desi-red sizeland shape, a model boot which, after curing, is removed from the lastand electroplated by a well-known procedure, thus providing a metallicshell from whose interior the model boot s removed, and which thenconstitutes a hollow mold whose interior is an exact replica (but inreverse) of the external surface of the model boot. This shell or moldis then employed in the manufacture of plastic boots for example, by theso-called slush molding process.

Since the wall of the moldy (that is to say, the metallic shellresulting from the electroplfating operation) is desira-bly thin, so asto facilitate the curing of the boot which is molded therein (a wallthickness of the order of l/a of `an inch being customary) and since themodel boot must be withdrawn from thel shell without danger ofdeforrning the latter, the model boot must be of a material such that itmay readily be deformed in response to the application of a forceinsuiiicient to bend the shell. For example, the model boot may be madeof textile fabric frictioned with rubber. When this easily deformablemodel boot is suspended in the electroplating bath, it must be weightedtoV prevent it from Heating, fcrinstance by lilling it with water, but.because of its inherent ilexibility, the internal weight tends toystretch it and cause it to sag-one common result being that its solebulges downwardly. Under such conditions, the shell, which is producedby the electroplating operation, is not an accurate repli-ca of themodel boot (as the latter was formed on the last) and when a plasticboot is made by the use oi this mold, the sole of the boot is downwardlyconvex instead of being substantially flat as it should be;

lThe principal object of the present invention is to provide a methodwhereby a mold, made by electroplat-ing a exible model boot, may be madeto have the accuracy of shape andV dimensions of the model boot as thelatter was made on tl e last. Other and further objects and advantagesof the invention will -be pointed out` in the following detaileddescription and by reference to the accompanying drawings wherein:

Fig. l is a diagrammatic side elevation of `a last of a size and styleappropriate for use in making a boot of the desired type;

Fig. 2 is a diagrammatic side elevation showing a model boot as made onthe last shown in Fig. l;

Fig. 3 is a diagrammatic side elevation showing the model 'boot of Fig.2 removed from the last;

Fig. 4 is a transverse section on the line 4-4 of Fig. 3;

Fig. 5 is a diagrammatic side elevation of a last of the same style `asthat of Fig. 1, but of a size smaller than the last of Fig. 1;

Fice

Fig. 6 is a diagrannnatic side elevation showinga hollow form or support`as made on the last of Fig. 5';

Fig. 7 is a diagrammatic side elevation of the form or support of Fig;6, removed from the last;

Fig. 8 is a diagrammatic transverse section onA the line 88 of Fig. 7;I

Fig. 9 is a vertical transverse section of they line 9-9 of Fig. 7 andshowing a plug filling thev top of the form;

Fig. 10 is aV diagrammatic side elevation, partly broken away, showingthe form of Fig. 7 disposed within the model boot of Fig. 2;

Fig. ll is a diagrammatic elevation showing the assembly of Fig. l0suspended in an electroplating tank;

Fig. 12 is a vertical front-torear section illustrative of the resultsof the electroplating operation;

Fig. 13 is a transverse section on the line 13-13 of Fig. 12, showingthe metallic shell resulting from the electroplating, with thek modelboot inside the shell and the form or support inside of the model boot;

Fig. 14 is a View similar to Fig. 13, but illustrating the step ofcollapsing the form preparatory to removing it from the model boot;

Fig. 15 is Ia similar transverse section illustrative of the step ofycollapsing the model boot, preparatory to removing it from the metallicshell or mold;

Fig. 16 is a front elevation of a last, which may be used instead of thelast of Fig. 1, in making the model boot;

Fig. 17 is a transverse section through a mold resultant from the use of*the last of Fig. 18;

Fig. 18 is a' diagrammatic transverse` section showing a mold, initiallymade by the use of a last like that of Fig. 1, but which has beenreshaped, by hammering, to a configuration like the mold of Fig. 17;and,

IFig. 19 is a transverse section illustrating the form of anotherconstruction.

Referring to the drawings, the numeral 20 designates a last lwhich maybe a conventional metal last, such as is used in the preparation ofrubber footwear, the last 20 being of that size and style which would beemployed in making a rubber boot of the style and size of the plasticboot which. it is intended to manu-facture. Having provided this last,therer is then built up on this last, for example by dipping or otherusual rubber footwear procedure, a model boot 21, whose wall is oftextile material frictioned with rubber and suiiciently thin to insurethe desired flexibility. Having built up this model boot on the last 20,the assembly is subjected to a vulcanizing operation, such. ascompletely to cure the model boot 21, whereupon the. latter is strippedfrom the last and turned right-side-out, `it necessary, so that theresultant model boot appears as shown in Figs. 3 and 4.

Another last 23 (Fig. 5) is now selected, which if the form -to be madethereon is initially to have a thick wall, may be a size smaller thanlas-t Ztl,` otherwise it may be of the same size and style as the last20 (or the latter may be used if desired) and upon this last, there isbuilt up a hollow form or support 24 (-Fig. 6) in the same general wayas the model boot, if desired.v This form or support is then desirablycoated or sprayed on its exterior surface with any suitable stiifeningmaterial which sets andv forms. a hard crust; for example, the materialemployed for this purpose may be ordinary shellac. When this coating hashardened, its surface is `smoothed as, for example, by sandpapering, Inbuilding up this form upon the last, it may be made to include a sole orother bottom portion or not, as desired. This form 24 is slit downwardlyfrom its top along one side, a-s shown at 25, and removed from the last,the resultant form being shown in Fig. 7. This slit 25 need not, in allcases, extend down to the lower edge of the form, since the lower partof the fonn may usually be separated 3 and withdrawn from the model booteven though the slit extend only part-way down, for example, to itsankle portion. Before removal from the last, this form is desirablytrimmed so that its lower edge 25X (Fig. 9) is ilush with the bottom ofthe last on which it was made. After removing the form from the last,the slit in its side may be closed temporarily, for example by anadhesive tape T (Fig. 19), and the interior of the form electroplated toprovide a thin, waterproof interior metallic shell E (Fig. 19). Such aninterior shell is advantageous in preventing the form from becomingsaturated with water during subsequent operations. If desired, the form,as built up on the last, may be removed therefrom without applying theaforesaid external stiiening coating, and the interior electroplated toform the internal shell as above described, said shell, in this event,being relied upon to impart the desired stiifness to the form. Afterthis shell E has been formed, it will ordinarily be slit downwardly inregistry with the slit 25.

A form made, as just described and comprising a stili inner metallicshell, may, if .it include a bottom as well as an upper portion, beemployed as a last. A last so made is much cheaper than a last made bycustomary methods and is very light in weight and thus easy to handleduring boot-manufacturing operations. Since this form is open at top andbottom and is slit downwardly along its side as at 25, it may bepartially collapsed so that the margins at opposite sides of the slit 25may be overlapped, thus reducing the transverse dimensions of the form;however, its natural resiliency tends to expand it to the dimensionswhich it had when formed on the last. Desirably, before using this form,an annular plug 27 (Fig. 9) of rigid material, for example wood, isinserted and removably secured in the open top of the form, the plugbeing of a size and shape such as to keep the upper end of the formfully distended. This plug has a hole for the admission of water, andmay be provided with a hook for use in suspending the form in theelectroplating bath.

Preparatory to the electroplating operation, this form 24, with the plug27 removed, is partially collapsed and disposed within the model boot21, as illustrated in Fig. l0, the resiliency of the form causing it toresume its normal shape, Athe plug 27 then being put in place. As abovedescribed, form 24 may or may not have a sole portion, but in eitherevent, it is coextensive with and contacts the model boot upper at allpoints. When thus installed within the model boot, the form holds thelatter fully distended and prevents it from collapsing or sagging. Thisassembly is then suspended within the electroplating tank 29, as shownin Fig. 11, by means of the suspension element 28 which is attached tothe plug 27 in the upper end of the form 24 With such an arrangement,the form 24 supports the entire weight of the model boot and suchweighting material as may be placed within the model'boot. A deposit ofany desired metal is now plated, by any customary procedure onto theexterior of the model boot, the latter having iirst been coated with asuit-able -film of conducting material, for instance a conventionalgraphite preparation, the plating operation being continued until ametallic shell 30 (Fig. l2) of the desired thickness (for example 1/2 ofan inch as above suggested) has been built up on the exterior of themodel boot 21. It Will be evident that by this operation, the interiorof the shell will have a contour which is an exact replica of theexternal contour ofthe model boot 21 except that it will be in reverse.When the metallic shell has been built up to the desired thickness, theassembly is taken from the plating tank, the plug 27 is removed from thetop ofthe former -24 and the latter is partially collapsed (as shown inFig. 14) by overlapping its margins at opposite sides of the slit 25 sothat it may be separated from the model boot 21. The former may now bewithdrawn from the interior of the rubber model. The rubber model maynow be twisted or collapsed (as illustrated in Fig. 15) to separate itsouter surface from the inner surface of the metal shell and is thenwithdrawn from the metallic shell 30 Without requiring the applicationof force suflicient to deform or damage the shell. The resultant mold,consisting of the metallic shell 30, is now ready for use in the moldingof boots, each an exact replica of the model boot 21.

While the above mode of preparing the form or support 24 for the rubbermodel is desirable, it is contemplated that the form may be made inother ways, for example, the last employed in making the form may besmoothly covered withv cotton stockinet and then coated or dipped with astiifening plastic of any well-known kind. After this coating has beencured, the stockinet is stripped from the last and then slit down oneside, as above described, if it be 'too stii to permit it to becollapsed without lso slitting it. If, when the form has been strippedfrom the last, it is not stiif enough, it may be sprayed or coated inits interior with any' suitable stiffening agent, for example shellac,or electroplated internally by any known procedure as `above suggested,thereby to increase its wall thickness and stiffness.

Forms made as above suggested may, if desired, be employed as displaysfor boots made on lasts like that used in making the forms or as lastsfor use in making boots.

While, according to the above method, the mold is of metal, made byelectrolytic deposit on the rubber model, it is contemplated, as withinthe broader scope of the invention, to make the mold by coating ordipping the rubber model, while held fully distended by the interiorform, so as to deposite a layer of plastic materi-al of any suitabletype on the exterior of the rubber model which, when cured, forms a hardand stiff shell. Such a Shell, after the form and rubber model have beenremoved, may serve for molding boots of a plastic material whichrequires only a low heat and a short period of curing.

EWhile as labove illustrated and described, the molded boot, as made, isof conventional type and like one which is built up on a last ofordinary construction, it is contemplated that the last on which themodel is made may be of the type disclosed in the co-pending applicationof George H. Bingham, Jr., Serial No. 713,433, filed February 5, 1958.The last therein described is shaped to provide certain areas whichimpart to the resultant boot top a contour such as to facilitate theattachment, by adhesive means, of a textile tape or tapes to theinterior surface of the upper, for example to facilitate the attachmentof the tapes of a slide fastener.

Thus, as shown in Fig. 16, the last 20P- has, at its front, a surface Fwhich is of uniform width from end-to-end and of uniform contour at anyhorizontal section, for example rectilinear in horizontal section, andof a width substantially equal to the total width of a slide fastener.The result of using such a last in making the model boot is that themold made from said model boot is like the mold M (Fig. 17) having theinterior surface F2 substantially like the surface F of the last 20a,but in reverse.

A mold having a similar interior surface may be made by taking a mold,prepared as above, by the use of the last 20 of Fig. l, and after themodel boot has been withdrawn from the mold, placing the latter (asshown at f30 Fig. 18) on an anvil A having an elongate face F3 of thedesired contour, and hammering the mold, for example with a rubberhammer, to conform the convex front portion R of the mold (as indicatedin dotted lines) to the shape of said face of the anvil.

Having prepared a hollow mold or container, as above described, it isdesirable to prepare, what is conveniently `called a prototype boot, bythe use of said mold, and to place the prototype in storage, to form asample or standard, so that if the mold be injured, destroyed, or

5 discarded, exact replicas of ythe mold can be prepared at any time bythe use of the prototype.

While certain procedures have been described as embodied in the presentinvention, it is to be understood that the invention is broadlyinclusive of any and all modiiications falling within the scope of theappended claim.

I claim:

AFor making a unitary, seamless, hollow mold having Walls of minimumlthickness consistent with use of the mold in forming a boot by theslush molding process, that method which includes the steps of providinga exible model boot, coating the boot with a ilm of conductive material,weighting the boot, with a material adapted to be poured into and fromthe boot, for `submerging `the boot in an electroplating bath,electroplating the boot for forming the mold, and removing the boot fromthe mold; in further combination with the intermediate steps ofproviding a hollow supporting form of stily resilient material having anormal size and shape corresponding to the inner surface of the boot,said form being adapted to be collapsed for inserting and withdrawingthe form from the boot, inserting the stiiiiy resilient hollow formwithin the boot, prior to the weighting of the boot, the inserted formtending to expand so as to contact the inner surface of the boot upperat all points and thereby holding the boot fully distended and withoutdistortion while the boot is submerged in the electroplating bath, yandcollapsing and withdrawing the form -as a unit from the boot prior toremoval of the boot from the mold.

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